Telephone switching system employing repetitive impulsing



' Dec. 2, 1952 w. A. MTALTHANER TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 14 Shgets$heet 1 INVENTOR W A. MALTHANER ATTORNEY Dec. 2, 1 52 w. A. MALTHANER TELEPHONE SWITCHING SYSTEM EMPLOYING REPEJTITIVE IMPULSING Filed June 29, 1948 14 Sheets-Sheet 2 AA VVY AAA 4 7T ORA/11' V Dec. 2, 1952 w. A. MALTHANER 2,520,399

TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 l4 Sheets-Sheet 3 FIG. 4

FIG. 5 FIG. 7 FIG. 9 F/G. FIG 6 FIG 8 FIG /0 FIG /2 FIG. /.3 FIG. /4 F/G. /5 FIG. /6

FIG. 5

lNVENTOR W A. MAL THANER A T TOR/V5 V Dec. 2, 1952 w. A. MALTHANER 2,

TELEPHQNE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 14 Sheets-Sheet 5 lNl/ENTOR W A. MALTHANER A TTORNE Y TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Dec. 2, 1952 w. A. MALTHANER l4 Sheets-Sheet 6 Filed June 29, 1948 we? 3K INVENTOR W A, MALrHA/vgR B JA ATTORNEY Dec. 2, 1952 w. A. MALTHANER 2,620,399

TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 l4 Sheets-Sheet '7 RE G/S TE R CONN.

TERT. CONN.

INVENTO/P W A. MALTHANER A TTORNEV Dec. 2, 1952 w. A. MALTHANER 2,620,399

TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 14 Sheets-Sheet 8 lo o /0 24 lNVENTOR M. A. MAL THAN/5R Z/JLMZ A-T TORNEV Dec. 2, 1952 w. A. MALTHANER 2,620,399

TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 14 Sheets-Sheet 9 B W A. MALT/IANER ATTORNEY Dec. 2, 1952 w A. MALTHANER 2,620,399

TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING l4 Sheets-Sheet 10 Filed June 29, 1948 IIHI- F/G. l2

'11 INVENTOR W. A. MALTHANER BWIAZZWXW- ATTORNEV 14 Sheets-Sheet 11 "Ill w. A. MALTHANER TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULS ING Dec. 2, 1952 Filed June 29, 1948 IEWN INVENTOR 'W A. MALTHANER Q GR Dec. 2, 1952 w. A. MALTHANER TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING 14 Sheets-Shet 12 Filed June 29, 1948 Dec. 2, 1952 w. A. MALTHANER 2,620,399

TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 I 14 Sheets-Sheet 15 M/l/ENTOR W A. MALTHANER 8V A TTORNE V FIG. /5

Dec. 2, 1952 w, A, MALTHANER 2,620,399

' TELEPHONE SWITCHING SYSTEM EMPLOYING REPETITIVE IMPULSING Filed June 29, 1948 14 Sheets-Sheet 14 INVENTOR W A. MAI. THANER A TTORNEV Patented Dec. 2, 1952 TELEPHONE SWITCHING SYSTEM EMPLOY- ING REPETITIVE IlYlPULSING William A. Malthaner, Summit, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 29, 1948, Serial No. 35,925

29 Claims. 1

This invention relates to an improved telephone switching system for automatically establishing telephone transmission paths between subscribers stations under control of equipment in each of the subscribers stations.

It is an object of the present invention to provide improved switching and control circuits, methods and equipment for providing improved telephone service.

In telephone switching systems in the prior art, it has been the practice to provide each subscriber station with a telephone transmitting and receiving device as well as a calling device which is usually called a dial or dialing device. While a dialing device is almost universally used in tele phone switching systems at the present time, nevertheless, it has certain drawbacks or disadvantages.

In the first place it requires an appreciable time for the subscriber to actuate the dial, and

thereafter, it requires an appreciable interval for the dial to return to normal for each digit during which time the pulses representing the magnitude of the digit are transmitted to the central office. Thus, the time required by the subscriber is extended by the time required to transmit and record the pulses to the central ofiice. Likewise, the time required to receive and record the called subscriber's number at the central office is lengthened by the time required by the subscriber or attendant to actuate the dial between each of the digits.

Furthermore, in switching systems in the prior art, signal currents representing the called subscribers station designation or number are transmitted to the central station but once. In addition the subscriber has no way of checking the accuracy of the number transmitted or determining the portion of the number actually transmitted to the central oifice'with the result that in case of error by either the subscribers or by the central oifice equipment, a wrong number will re sult.

2 thus materially reduce the holding time of the expensive control-ling equipment. In this mannor the efficiency of the use of the equipment is increased with the resultant lower cost of establishing each connection there-through.

Another object of the invention is to provide improved methods, circuits and equipment for more accurately establishing connections through the equipment, first, by arranging the subscribers calling apparatus so that the subscriber may set up the number of the called party on the equipment and then check the setting of the equipment before placing the call.

Another object of this invention is to improve methods, circuits and apparatus for more accurately establishing connections through switching equipments by repeatedly transmitting the number to the central oflice where the received number may be checked to insure that it has been properly received. If the number is improperly received the recorded number will be disregarded and the next time it is transmitted from the subscribers station it will again be recorded and if properly received employed to control the establishment of a connection. Then, after all the selections have been made to establish a path through all the switching equipments to the called subscriber's station, the calling device at the calling subscribers station is automatically disconnected from the line to the central station and the telephone transmitting and receiving equipment connected to the line.

In United States Patent 2,232,398 granted to Lunclstrom on February 18, 1941 a rectifier bridge is employed to interconnect the various signaling and talking apparatus to the line and to reduce the effects of these devices upon each other. However, the devices are never disconnected from the line and each introduces transmission loss to the currents of the other and still may produce other undesirable interference.

A feature of this invention relates to switching apparatus for removing either one or the other of these devices from the line at all times thus preventing all interference between them.

A feature of the invention relates to a switching system incorporating switching and control circuits and apparatus wherein all idle paths through the switching apparatus are marked and electronic tubes arranged in lock-out circuits employed to rapidly test all the thus marked idle paths through the switching equipment and select one of them. Such switching and control circuits and apparatus are disclosed in a patent application of E. Bruce and N. I. Hall filed on 3 October 29, 1947, Serial No. 782,704, now Patent No. 2,582,959, the disclosure of which is incorporated herein by reference to the same extent as if fully set forth herein.

Another feature of this invention relates to the use of the subscribers calling apparatus and equipment such as shown in a patent application of Parkinson Serial No. 35,930, filed on June 29, 1948, now Patent No. 2,597,898, wherein each of the digital positions of the calling subscribers number is represented by two pulses variably spaced in time one from another and wherein the time elapsing between the two pulses representing each digital position of a number represent the magnitude of the respective digits or the character or characteristics of the letter of the alphabet or another symbol in the respective positions in the number. The above-identified application of Parkinson is hereby incorporated herein by reference to the same extent as if fully set forth herein.

Another feature of the invention is directed to equipment for receiving such pulses of the type disclosed in a patent application of Malthaner- Newby-Vaughn Serial No. 35,924, filed on June 29, 1948, the disclosure of which is made a part hereof by reference as if fully set forth herein. Novel features disclosed herein but not claimed herein may be claimed in the above-identified applications.

Another feature of the invention relates to control circuits for coordinating the operation of all the circuits and arranging them to cooperate one with another in the rapid and efficient use of the subscriber's time and also the time of the control equipment at the central office, thus materially reducing the time required to and cost of establishing each connection through the switching equipment.

Briefly, in accordance with an exemplary embodiment of this invention circuits and equipment are arranged so that a subscriber in initiating a call first sets his calling device or equipment in accordance with the number or other symbols designating the called subscribers station. Thereafter, the subscriber will place his call by picking up the receiver or handset in the usual manner at which time the subscribers calling equipment is connected to and maintained connected to the line extending to the central station. The subscriber in initiating a call will set into operation the equipment at the central station for extending his line to call receiving and recording equipment. When this connection is established pulses transmitted from these subscribers calling equipment will be received, re-,

corded and checked by the receiver. Thereafter, the recorded pulses are employed to selectively control switches to extend the subscribers connection to the called subscribers line. When the selection of the called subscribers line is completed, the electrical condition or conditions applied to the calling subscribers line are changed which change disconnects the subscribers calling equipment from the line and connects the telephone transmitting and receiving equipment thereto. If the called line is busy the subscriber will then receive a busy tone whereupon the subscriber will hang up. If the subscriber desires to call at a later date it should be noted that his calling equipment is already set to the desired number so that he may again place the same call by again merely lifting the receiver from the hook or picking up the handset, as the case may be, whereupon the circuits again operate as described above. If the called line is idle ringing current is applied to it. When and if the subscriber answers the ringing current is removed from his line; the proper electrical condition applied to the line to cause the telephone transmission apparatus at the called subscribers station to be connected to his line; and, the communication path between the subscribers established so that they may communicate with each other. At the completion of the call the circuits and apparatus are all returned to their normal or idle condition.

The foregoing and other objects and features of this invention, the novel features of which are specifically set forth; in the claims appended hereto, may be more readily understood from the following description when read with reference to the attached drawings in which:

Figs. 1 and 2 show in outline form the various elements of an exemplary system embodying the present invention and the manner in which they cooperate one with another;

Fig. 3 shows the manner in which Figs. 1 and 2 are arranged adjacent one another to illustrate the exemplary system set forth herein;

Fig. 4 shows the manner in which Figs. 5 to 16 are arranged adjacent one another; and

Figs. '5 to 16 show in greater detail the various elements and the manner in which they cooperate to form an exemplary system embodying the present invention.

It should be noted that the term calling as used in this disclosure is not limited to completed calls. The term call as used in the telephone art includes completed calls, calls in which the busy tone is received, and calls for which the ringing tone is received but no answer is received from the called party. The term call as used in this disclosure, and in the automatic telephone switching field in general, may generally be said to include any operation by the calling party in which he causes his telephone to make connection with, and to control the operation of, the switching apparatus in the central oflice.

Twelve subscriber stations are shown in Fig. 1. one subscriber station is illustrated generally by the'reference numeral H0 and another is shown at I20. The other subscriber stations are indicated by rectangles such as H5, H6, H1, H8, H9, etc. are actually shown in the drawing it is to be understood that any suitable or desired number may be provided Without departing from the present invention or the exemplary system set forth in detail herein disclosing such a system.

The subscribers station shown generally at I I0 is connected to line H4 extending to the central switching station. This subscribers station comprises a telephone set H2 which includes a transmitter and receiver and other equipment normally provided as part of a telephone subscribers phone set. The subscribers equipment H0 also includes a dial or calling device H3. While this invention is not limited to the exemplary system set forth herein, the system is arranged to cooperate with a calling device of the type set forth in the above-identified application of Parkinson.

The subscriber station H0 also includes a polarized relay H which is employed to connect either the telephone set H2 or the calling device H3 to the transmission line H4 extending to the central switching station. Relay H l is controlled over the line I I4 extending to the central switching station as will be described hereinafter.

While only twelve subscriber stations The subscriber station I20 is similar to subscriber station IIO. Telephone set I22 is provided and includes a telephone transmitter and receiver which is employed, in the usual manner to talk over the telephone. Likewise, calling set I23 is also provided tosignal and controlthe central switching station to set up a desired telephone transmission path to the called subscriber station.

In. the case of the subscriber station I20 however, a neutral relay I2 I- is provided instead of. a polar relay as shown at station IIO. When a neutral relay I2I is employed it is necessary to provide rectifiers I24 and I25 to cause the relay I2 I as well as the other subscriber station equipment and central station apparatus" to respond in the proper manner to. the various circuits as will be describedv hereinafter.

The switching equipment at the central station comprises a plurality of cross-bar switches and associated controllin circuits as well as circuits responding to the signal set out by the calling devices II3, I23 and similar devices at the other subscribers stations. A typical cross-bar switch is described in detail in United States Patent 2,021,329 granted to Reynolds on Novem'- ber 19, 1935, the disclosure of which patent is hereby made a part of the present application as if fully set forth herein.

As set forth in the above-identified application of Bruce-Hall, Serial No. 782,704, filed 0ctober 29, 19.47 cross-bar switches are arranged in groups. Briefly, as shown in, Fig. 1 cross-bar switches I30 and I III'show two switches. of one primary group of cross-bar switches, while switches I50 and I50-represent' two switches of a second primary switch group. Only two switches in each of two primary switching groups, are shown in the drawing. It willbe readily apparent that the usual; arrangement will' comprise up to ten primary switches andmultiples thereof'in one primary group. A many primary groups may be provided as desired. Switches I and I80 represent two switches of a secondary group of switches. Likewise, switches I90 and I95 represent two other secondary switches of a second secondary group ofswitches. Usually, as many groups of secondary switches will be provided as there are primary groups and secondary groups may comprise up to and including tenswitches. Switches 2 I0 and 220 represent two switches of a group of tertiary switches. As'before, any suitable number of switches may be-included in any suitable number of groups of tertiary switches. In addition to the foregoing switches, a. switch 230 or a group of such switches are employed to connect the register circuit to receive the signal pulses designating the called subscriber station which have been transmitted over the subscriber's line to the central switching station. As set forth in. the above-identified Reynolds patent each of the cross-bar switches is provided with a group of select magnets sometimes called horizontal magnets and another group-of magnets called vertical or hold magnets. Operation of one select magnet and one hold. magnet causes contacts at the cross-points of the operated. select magnet andv hold magnet to be actuated to establish av transmission path through that switch. Then after, the select magnetmay be released without releasing the cross-point contacts. As long as the hold magnet remains operated the operated cross-point contacts remain operated. The hold magnets are designatedH in the, drawing and the select magnets S.

In addition, a cold cathode discharge tube is associated with each of the select magnets and each of the-hold magnets in the primary switches. Other gas tubes and relays notshown in Fig. 1 are described in greater detail in the above-identified copending application of Bruce-Hall and are employed to aid in the control and seizing of idle paths through the switching system.

Eachof the subscriber lines, such as H4 and I26 and the subscriber lines associated with others of the subscriber stations shown in Fig. 1, terminates in a vertical column of contacts on theprimary line switches. Thus, line II I from the subscriber station I I0 terminates in the vertical associated with the hold magnet and control equipment I33. The subscriber line I26, extending to station I20, terminate in the vertical associated with the hold magnet and control equipment I32. Likewise subscriber stations I I6 and I II, which are not shown in detail terminate, in vertical columns of switch I 40. The other subscriber lines likewise terminate vertical columns of the above enumerated or other primary cross-bar switches. If desired, the subscriber's line could be connected to horizontal rows of contacts providing other minor changes are also made in the circuits.

In addition to the foregoing elements a group of circuits frequently called junctor circuits such as circuits H5, I85, I86, I81 are provided to aid in the establishment of transmission paths through the system as well as to supply talking battery after the connection has been selectively established. Likewise, the common release circuit 233 is provided to release the control circuits after each communication path has been established through the switches so that these control circuits may be employed to establish another path through the switching system. As is well understood, the release of the control equipment employed to establish a, path through the system does not release the communication path established by these circuits. Instead, the paththrough the system is maintained under control of the subscribers as will be described hereinafter. Likewise, register control circuits 234 and 235 and associated pulse receiving and storing equipments 236 and 231 and 238 and 239 respectively have been provided to receive the pulses representing the called subscriber station and to select the path over which the call is to be routed to the called subscriber station.

As set forth in the above-identified application of Bruce-Hall all the paths which are idle between the subscriber's lines, or the primary switches, and a transmission or junctor circuit within or extending from within to without the central switching station are marked and then one idle path selected by gas tubes arranged in lock-out circuits.

A pulse receiver 236 comprises the elements set forth in the above-identified application Malthaner et al. In addition, the called subscriber's designation or number is stored on a groups of relays connected to tubes in the comparison circuit 23'! as shown in Fig. 2. If. it is desired, the

' arrive at the central switch place.

nation of the called station and then this device is connected to the subscriber line H4 extendin to the central switching oflice. Initiation of the call by the subscriber also initiates a discharge through the tube associated with this line in the control equipment I33. This tube in turn causes other circuits to be operated which in turn causes start relay 25| in the release and preference circuit 233 to operate. The operation of relay 25l applies potential to the conductor 256. This conductor extends through the contacts of a relay in each of the idle register circuits 234 and 235 and such other register circuits as may be provided to lead 253. If none of these register circuits are idle the potential applied to lead 259 is not applied to lead 253. However, when one of the registers becomes idle potential from the conductor 256 will be applied through contacts of the -S relay of the register circuit which is idle. The circuit of lead 253 then extends through break contacts of the off-normal relays of junctors I15, I85, I86 and I81. As shown in Fig. 1 the junctor circuits are arranged in two groups, junctors I75 and I85, forming a part of one group, junctors I 66 and I8! forming a part of another group. While only two junctors are shown in each of the two groups, it to be understood that any suitable number of junctors may be provided and may be arranged in any desired number of groups. However, for purpose of illustrating a specific embodiment embodying the present invention, two junctors in each of two groups are shown in Fig. 1. The circuit from lead 253 thus extends through break contacts of relays of the two junctors of the first group, namely, H5 and 185 to conductor 255 and through similar break contacts in junctors I86 and I6! to lead 254. If none of the junctors are idle the circuits are incomplete with the result that no potential is supplied to either of leads 255 or 254. However, as set forth it is assumed that at least one junctor is idle. If one junctor in each group is idle then the potential from lead 253 is applied to conductors 254 and 255. Conductors 254 and 255 extend through break contacts on relay I96 to conductors 256 and 251. Conductors 256 and 25! extend to break contacts of the hold magnets associated with the secondary cross-bar switches. Thus conductor 256 extends to the break contacts of the hold magnets associated with switches I86 and I95 while conductor 25l extends through the break contacts of the hold magnets associated with switches H0 and I96. If the various vertical circuits of these switches are busy the hold magnet will be operated and thus will interrupt the connection from leads 256 or 257 through the break contacts of the operated hold magnets. However, if these elements are idle the hold magnet will be released and will extend the circuit from leads 256 and 251 to elements in the tubes associated with the horizontal rows of contacts of the primary cross-bar switches. Thus the potential from the conductor 25!! and the start relay 25I of the release and preference circuit 233 extends over all idle paths between the primary line switches and the junctor circuits, providing of course there is an idle junctor, and an idle register for use in establishing a call from the subscriber station l l6.

Inasmuch as the subscriber line H4 terminates on switch 130, it will be necessary to operate one of the tubes associated with the horizontal magnets in switch I30. In order to accomplish this it is also necessary to prevent an attempt to establish a call simultaneously through some other cross-bar switch, so the tubes associated with the horizontal magnets of all of the primary line switches are arranged in a lock-out circuit such that only one tube can operate at a time. Upon the operation or conduction of one of the tubes individual to a horizontal row of contacts of switch I30, a path is selected and then established through the switching system to a junctor circuit such as and from the junctor circuit through the register connector switch or switches 230 to the register circuit 234 and pulse receiving apparatus 236 and 231. Negative battery is connected to the tip of the line extending to the subscriber station and ground is connected to the ring. The polar relay III at the subscriber station III] has its windings connected to the line so that its armature will remain in contact with the righthand contact as shown in the drawing when battery and ground are connected to the subscriber's line in this manner thus connecting and maintaining the calling or dialing equipment I [3 connected to the subscriber line.

Upon the establishment of a communication path from calling equipment H3 at the subscriber's station I II) to the register equipment, say for example, 234, signaling pulses are transmitted from the calling equipment H3 to the register control circuit 234 and then over the tip and ring conductors designated by 213 in Fig. 2 to the receiving equipment 236. This equipment comprises a low-pass filter 260 and a limiter or gain regulator 26l, converter circuit 268, start circuit 264, a counting circuit 262 for checking the total number of pulses received rep resenting the called subscribers station designation, digit steering or stepping circuit 265, and one or more counting circuits 266.

When desired or necessary, steerin circuit 265 will be arranged to direct or steer the pulses representing the magnitude of the digit in each digital position of a subscribers designation to the respective counting and storing circuits. Thus, in the case of an eight digital position designation, the steering circuit will direct or distribute pulses representing the magnitude of the digit in each of the digital positions of the proper one of eight countin and storing circuits.

The digital counter 266 is shown in the drawing. This is shown merely by way of example showing the position of the counting and storing circuits in the system and the manner in which the counting circuits cooperate with the other circuits and elements of an exemplary system. This counter represents any suitable or necessary number of orders of binary counting circuits from one to a maximum number of digital positions in a complete subscribers designation. When it is necessary to set a call up through several exchanges in tandem it is not always necessary to employ all of the digits of the called subscribers number at all of the exchanges. For example, the office code or name is all that is frequently employed in setting a call up through the first and intermediate exchanges whereas the last portion of the called subscribers number is the only portion used in the final switching station. It is to be understood however that the number of digits employed at any switching station is a function of the size of a switching station as well as of the complexity of the switching required at that station. When, as in the usual case, it is only necessary to use a. portion of the digital positions of a. subscribers designation at any one switching 9 center the counting circuit 266 represents that portion which it is necessary or desirable to use at any one switching center. This porton may comprise the first one or more digital positions, one or more intermediate digital positions or one or more of the final digital positions of the sub scribers station designation. The counter as shown in the drawing may be employed to count and store pulses representing the magnitude of the digits in either consecutive or non-consecutive digital positions of the complete subscribers designation.

The output of the counting circuits is applied to a plurality of vacuum tubes and associated storing relays shown with the rectangle 231. The above-described circuits operate in substantially the same manner as described in the aboveidentified application of Malthaner-Newby- Vaughan Serial No. 35,924, filed June 29, 1948.

A checking circuit 262 recognizes the complete set of pulses representing a subscribers station designation. If the complete set of pulses as received at the switching station does not comprise the proper number of pulses, the equipment at the receiver is recycled by the recycle circuit 263 and the next series of pulses is employed to control a further operation of the system. If a proper number of pulses has been received the position or setting of the binary counters 26B is transferred to the relays in circuit 231 and a signal transmitted to the register control circuit 234 whereupon the lock-out relay 269 is operated through the gas tube 210 which is arranged in a lock-out circuit including other similar tubes and other registers such as tube 212 and register 235. The lock-out circuit is provided to prevent more than one register from seizing or using the number group circuit 249 at a time. When relay 269 operates it connects positive battery to three of the leads extending to the number group circuit 240. As set forth in the above-identified application of Bruce-Hall, Serial No. 782,704, filed October 29, 194.7, the application of positive potential to three leads extending to the number group circuit selects the line circuit tube associated with the called subscribers line and causes a discharge to be initiated through this tube. It should be pointed out that prior to the operation of lockout relay 269 certain other circuits function to prevent the establishment of a connection from an incoming subscribers line to another junctor register connector and register circuit.

It is also evident that there are ten leads from each of the register circuits extending to the number group circuit for each digit which is desired or necessary to employ at the switching oflice. If, as described in the above-identified application of Bruce and Hall, it is necessary to employ only three digits, thirty leads will extend from each of the relays 269 and 2! I, and similar relays of all of the other register circuits which may, of course, include more than the two shown, to number group circuit 240. Upon the application of positive potential to one lead in the hundred group and one lead in the ten group, and one lead in the unit group, one of the conductors extending from the number group circuit 240 to the subscribers line tubes will have a sufliciently high positive voltage with respect to ground applied to it to initiate a discharge within the subscribers line tube associated with the called line if the called line is idle. The initiation of a discharge through this subscribers line tube identifies or marks the called partys line and causes the various circuits to function to establish a connection from the junctor to the called subscribers station. If the called line is busy a discharge is not initiated in the line tube associated with the called partys line and as a result the circuits time out and send a busy signal which is transmitted to the calling subscribers station and transmission apparatus.

In establishing connection to the called subscribers line when it is idle the register circuit 234 causes the operation of the horizontal or select magnet of the tertiary cross-bar switch associated with the junctor I15. The operation of this magnet causes the operation of additional relays, which relays apply a potential through the break contacts of the hold magnets associated with the tertiary cross-bar switch to which the junctor circuit H5 is connected. The circuit through the break contacts of these hold magnets then extends through winding of some of the select magnets of the secondary switches through operated contacts of relay I96 to conductors 256 and 251 which in turn extend through break contacts of hold magnets associated with the secondary cross-bar switches and then to the discharge tubes associated with the horizontal rows of contacts of primary cross-bar switches. These tubes associated'with the horizontal rows of the primary cross-bar switch are connected in a lock-out circuit arrangement which permits a discharge to be initiated through only one of these tubes whereupon one of the possible paths from the junctor I15 to the called partys line, as for example I20, is selected. Thereafter, the relays and magnets associated with this path are operated and establish a communication path from the junctor circuit I15 to the called subscribers line which may be I28 for example. Upon the complete establishment of the above-identified path, the release circuit 233 is set into operation which in turn causes the register 234 to be disconnected and relays of junctor circuit I15, to operate. The operation of relay I16 in the junctor circuit I15 reverses the application of battery and ground to the tip and ring conductors extending to the calling subscribers station III). This reversal of polarity causes the polarized relay III at the calling subscribers station I I!) to be actuated so that its armature will now make contact with its left-hand contact instead of with its right-hand contact, thus interrupting the circuit to the calling equipment H3 and connecting the telephone receiving and other transmission apparatus to the line extending to the central switching station. If the called line is busy, a busy tone will then be transmitted to and received by the receiving equipment forming a part of the transmission equipment designated by H2 in Fig. 1. The subscriber upon noting this condition will replace his instrument on the hook or cradle and if desired, place the call again at a later time.

If the called line is idle, ringing current will be applied to it upon the establishment of the connection through the cross-bar switches as described above and as set forth in greater detail in the above-identified application of Bruce-Hall Serial No. 782,704. When the called party answers, the ringing is stopped and the communication between the two stations established. Upon the termination of communication between the two stations, the station equipment as well as the switching equipment in the central exchange is all restored to its normal or idle condition and is then available for use in establish- 11 ing transmission paths for other calls in a manner similar to that described above.

While it is assumed that the call originates in station H for station (20 it will be readily .apparent that the call can be made in either direction from either of these stations to the other or from either of these stations to any other station. Likewise all of the stations can be similar to station H0 or to station I20 and the system will work in substantially the same manner as described above.

It is also evident that any suitable type of switching equipment may be employed at the central station in cooperation with the subscribers station calling device and the receivin circuit described herein and in the above-identified patents and applications made a part of the present application by reference.

An exemplary system embodying the present invention is shown in greater detail in Figs. through 16, inclusive. Fig. 5 shows two subscribers stations in detail and includes a third one 510. The two stations, one at 5!!) and the other at 550 are not identical. It is to be understood, of course, that all of the stations connected to the system may be of either type or they may be some of one type and some of another as shown in Fig. 5. The station 5! is provided with calling or dialing apparatus 52:1, telephone transmitting and receiving apparatus 5I2 and 5l3 and other types of equipment usually employed in telephone stations including the ringer 5I5, repeat or induction coil 5| I, switchhook contacts 514 and so forth. In addition a switching relay 5H5 is provided together with rectifiers 5H and 5I8. The above-mentioned devices are all well known in the art except perhaps the signal or calling device 520 which in the exemplary embodiment set forth herein is in accordance with the disclosure of the aboveidentified application of Parkinson.

The subscribers station 550 is provided with similar apparatus including a receiver 552, transmitter 553, switchhook contacts 554, ringer 555 and induction coil 55I, in addition, calling device 560 is provided which may be similar to calling device 520. As shown at station 550 a polar relay 556 is provided for switching from the telephone transmitting and receiving apparatus to the calling equipment and back. In addition, non-linear resistor devices 528 at station 510 and 566 at station 550 are provided to eliminate or greatly reduce clicks which may arise incident to the switching either at the subscribers station or the central ofiice. Additional subscribers stations are shown on Fig. 6 such as 632, 634 and 636 as well as other undesignated subscribers stations shown in Fig. 6.

It should be noted that the polar relay 55B is across the line in parallel with either the calling apparatus or in parallel with the voice frequency transmission and receiving apparatus and connects one or the other of these devices to the line and disconnects the other from the line. On the other hand relay 516 is in series with the talking apparatus when this apparatus is connected to the line but in parallel with the calling apparatus when this apparatus is in use. Relay 516 connects and disconnects the calling device 520 and the line but short-circuits or removes the short circuit from around the voice frequency transmission apparatus. Also when the voice frequency apparatus is in use the winding of relay 5l6, rectifier MB and the retardation coil are shunted by condenser 5l9 and rectifiers 5. Of course other equivalent ways of connecting the windings of the switching relays and their contacts may be employed when desired.

While the calling equipment at the subscriber's station as well as the receiving equipment at the central ofiice may be of any suitable type, the exemplary system described herein has been arranged to cooperate with the repetitive calling apparatus of the typedisclosed in the aboveidentified application of Parkinson. Likewise while any suitable type of telephone switching system such as panel systems or cross bar systems, which systems are Well known in the prior art, may be employed at the central switching station, the exemplary system embodying the present invention employs a switching arrangement of the type set forth in the above-identified application of Bruce and Hall.

As set forth in the above-identified application of Bruce and Hall and shown herein, the cross-bar switches employed to establish connections within and through the system have been divided into groups of primary switches, secondary switches, tertiary switches and register connector switches. Usually, a primary and secondary group of switches comprise ten levels or ten groups of primary switches and ten levels or ten groups of secondary line switches. Each pri- I mary level may include any number of switches. In the usual case one, two or three of the switches will have their horizontal rows or circuits connected together to form in effect one extended switch or switch level. Each switch on each level usually has ten horizontal rows which are connected together to form a circuit. The subscribers lines terminate on the vertical columns or groups of contacts of the primary line switches. Each horizontal row from each of the primary switches connects to a vertical group of contacts in a different one of the ten different secondary switches. The horizontal rows of the secondary line switches are connected to trunks or circuits extending to the different junctor circuits. On terminating calls the secondary horizontal rows are connected to vertical columns on a group of tertiary cross-bar switches, the horizontal rows of which are likewise connected to the opposite ends of the junctor circuits. A group of cross-bar switches are also employed to connect the register circuits to the calling line to receive the calling information as will be described hereinafter.

Figs. 6, '7, 8, 9, 10, 11 and 12 show sufiicient details of the switching system set forth in the above-identified application of Bruce and Hall to understand the manner in which it operates and cooperates with the other elements of the exemplary system set forth herein. Figs. 13, 14, 15 and 16 likewise show the details of certain portions of the disclosure of the above-identified application of Malthaner, Newby and Vaughan necessary to understand the manner in which this equipment operates and cooperates with the other portions of the exemplary system set forth herein.

When a subscriber 550, for example, is to initiate a call he will first set his calling device in accordance with the number or designation of the called station and then he will remove the handset from the cradle or lift the receiver from the switchhook which completes a direct-current circuit over the subscribers line 565, which connects start ground to the upper terminal of resistor 638 and thus applies potential conditions to control gap of tube -6l3 which causes a dis- 

